Method and apparatus for reducing inappropriate detection of lead-related noise

ABSTRACT

An implantable medical device detects conditions such as a lead failure which may result in oversensing a physiologic condition. In response, the IMD automatically adjusts sensing thresholds, such as the number of intervals to detection in order to mitigate the effect of oversensing in the delivery of extraneous therapy.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims priority and other benefits from U.S.Provisional Patent Application Ser. No. 60/883,234, filed Jan. 3, 2007,entitled “IDENTIFICATION OF OVERSENSING IN A MEDICAL DEVICE”,incorporated herein by reference in its entirety. Cross-reference ishereby made to commonly assigned U.S. patent application Ser. No.10/418,857, filed Apr. 18, 2003, titled “Method and Apparatus forIdentifying Cardiac and Non-cardiac Oversensing Using IntracardiacElectrograms” and U.S. patent application Ser. No. 11/115,607, filed May23, 2005, titled “Method and Apparatus for Identifying Lead RelatedConditions Using Prediction and Detection Criteria” which areincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus forautomatically identifying cardiac and non-cardiac oversensing by animplantable cardiac device and automatically adjusting detectionparameters to reduce inappropriate detection.

BACKGROUND

Implantable medical devices are available to provide therapies forrestoring normal cardiac rhythms by delivering electrical shock therapyfor cardioverting or defibrillating the heart in addition to cardiacpacing. Such a device, commonly known as an implantable cardioverterdefibrillator or “ICD”, senses a patient's heart rhythm and classifiesthe rhythm according to a number of rate zones in order to detectepisodes of tachycardia or fibrillation. Single chamber devices areavailable for treating either atrial arrhythmias or ventriculararrhythmias, and dual chamber devices are available for treating bothatrial and ventricular arrhythmias. Rate zone classifications mayinclude slow tachycardia, fast tachycardia, and fibrillation.

Upon detecting an abnormal rhythm, the ICD delivers an appropriatetherapy. Cardiac pacing is delivered in response to the absence ofsensed intrinsic depolarizations, referred to as P-waves in the atriumand R-waves in the ventricle. In response to tachycardia detection, anumber of tiered therapies may be delivered beginning withanti-tachycardia pacing therapies and escalating to more aggressiveshock therapies until the tachycardia is terminated. Termination of atachycardia is commonly referred to as “cardioversion.” Ventricularfibrillation (VF) is a serious life-threatening condition and isnormally treated by immediately delivering high-energy shock therapy.Termination of VF is normally referred to as “defibrillation.”

In modern implantable cardioverter defibrillators, the physicianprograms the particular anti-arrhythmia therapies into the device aheadof time, and a menu of therapies is typically provided. For example, oninitial detection of an atrial or ventricular tachycardia, ananti-tachycardia pacing therapy may be selected and delivered to thechamber in which the tachycardia is diagnosed or to both chambers. Onredetection of tachycardia, a more aggressive anti-tachycardia pacingtherapy may be scheduled. If repeated attempts at anti-tachycardiapacing therapies fail, a higher energy cardioversion pulse may beselected. For an overview of tachycardia detection and treatmenttherapies reference is made to U.S. Pat. No. 5,545,186 issued to Olsonet al., which is hereby incorporated by reference in its entirety.

Detection of tachycardia or fibrillation may also trigger the storage ofthe sensed intracardiac electrogram (EGM) for a period of severalseconds such that the EGM signals leading up to and during a detectedarrhythmia episode are available for downloading and displaying on anexternal programmer or other device for analysis by a physician. Suchanalysis aids the physician in monitoring the status of the patient andthe patient's response to delivered therapies. Occasionally,cardioversion or defibrillation therapies are delivered when the patientis not symptomatic. For example, the ICD may inappropriately detect atachycardia or fibrillation episode that does not exist and deliver ananti-arrhythmia therapy when it is not needed. Inappropriate arrhythmiadetections may cause a patient to experience painful, repeated shockswithin a short period of time. Anti-tachycardia pacing therapiesdelivered during normal sinus rhythm can potentially induce anarrhythmia in some patients. For these reasons, the delivery of atherapy in response to inappropriate arrhythmia detection isundesirable.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an exemplary implantable medical devicein which the present invention may be usefully practiced;

FIG. 2 is a functional, block diagram of the implantable medical deviceof FIG. 1, in which methods included in the present invention may beimplemented;

FIG. 3 is a flow chart of a method for delivering therapy in a medicaldevice, according to the present invention;

FIG. 4 is a flow chart of a method for detecting a lead-relatedcondition in a method for delivering therapy according to an embodimentof the present invention;

FIG. 5 is a flowchart of a method for determining whether an oversensingcriteria has been satisfied during detection of a lead-related conditionaccording to an embodiment of the present invention;

FIG. 6 is a graphical representation of a method for determining whetheran oversensing criteria has been satisfied during detection of alead-related condition according to an embodiment of the presentinvention;

FIG. 7 is a flowchart of a method for determining whether an oversensingcriteria has been satisfied during detection of a lead-related conditionaccording to an embodiment of the present invention;

FIG. 8 is a flow chart of monitoring impedance trends for the detectionand diagnosis of a lead-related condition according to an embodiment ofthe present invention;

FIG. 9A is a flow chart of a method for determining short-term andlong-term impedance trend parameters that may be included in anembodiment of the impedance trend monitoring of FIG. 8;

FIG. 9B is a graphical representation of hypothetical daily impedancedata generated according to an embodiment of the present invention;

FIG. 9C is a graphical representation of a method for determininglong-term maximum and minimum baselines according to an embodiment ofthe present invention.

FIG. 10 is a flow chart of a method of monitoring impedance to detect anopen or short circuit according to an embodiment of the presentinvention;

FIG. 11A is a flow chart of a method of monitoring impedance to detectinsulation degradation according to an embodiment of the presentinvention;

FIG. 11B is flow chart of a method for detecting lead insulationdegradation using non-parametric methods according to an embodiment ofthe present invention;

FIG. 12 is a flow chart of a method for monitoring trends in leadimpedance parameters to detect middle insulation degradation due tometal ion oxidation according to an embodiment of the present invention;

FIG. 13 is a portion of a stored electrogram showing near-field andfar-field pulses where there is an indication of a false positivenear-field pulse;

FIG. 14 is a portion of an electrogram showing near-field and far-fieldR-wave sensing pulses where there is an actual cardiac episode requiringtherapy;

FIG. 15 is a flow chart of a method for determining the presence ofoversensing in a method for of delivering a therapy in an implantablemedical device, according to an embodiment of the present invention;

FIGS. 16A and 16B are graphical representations of a determination of abaseline measure of a far-field signal according to an embodiment of thepresent invention;

FIG. 17 is a flow chart of a method for determining the presence ofoversensing in a method for delivering a therapy in a medical device,according to an embodiment of the present invention;

FIG. 18 is a graphical representation of maximum and minimum amplitudesof sensed events in a method of determining the presence of oversensingin a method for delivery of therapy in a medical device according to anembodiment of the present invention.

FIG. 19 is an illustration of an implantable cardiac stimulation devicecapable of pacemaking, cardioversion, and defibrillation incommunication with a patient's heart via three stimulation and sensingleads.

FIG. 20 is a flow chart providing an overview of one embodiment of thepresent invention for automatically reducing the likelihood ofinappropriate detection of noise in an implantable medical device.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of a pacemaker/cardioverter/defibrillatorand lead set of a type in which the present invention may usefully bepracticed. The ventricular lead includes an elongated insulative leadbody 16, carrying three mutually insulated conductors. Located adjacentthe distal end of the lead are a ring electrode 24, an extendable helixelectrode 26, mounted retractably within an insulative electrode head28, and an elongated coil electrode 20. Each of the electrodes iscoupled to one of the conductors within the lead body 16. Electrodes 24and 26 are employed for cardiac pacing and for sensing ventriculardepolarizations. At the proximal end of the lead is a bifurcatedconnector 14, which carries three electrical connectors, each coupled toone of the coiled conductors.

The atrial/SVC lead includes an elongated insulative lead body 15, alsocarrying three mutually insulated conductors. Located adjacent theJ-shaped distal end of the lead are a ring electrode 21 and anextendible helix electrode 17, mounted retractably within an insulativeelectrode head 19. Each of the electrodes is coupled to one of theconductors within the lead body 15. Electrodes 17 and 21 are employedfor atrial pacing and for sensing atrial depolarizations. An elongatedcoil electrode 23 is provided, proximal to electrode 21 and coupled tothe third conductor within the lead body 15. At the proximal end of thelead is a bifurcated connector 13, which carries three electricalconnectors, each coupled to one of the coiled conductors.

The coronary sinus lead includes an elongated insulative lead body 6,carrying one conductor, coupled to an elongated coiled defibrillationelectrode 8. Electrode 8, illustrated in broken outline, is locatedwithin the coronary sinus and great vein of the heart. At the proximalend of the lead is a connector plug 4 that carries an electricalconnector, coupled to the coiled conductor.

The pacemaker/cardioverter/defibrillator 10 includes a hermeticenclosure or housing 11 containing the electronic circuitry used forgenerating cardiac pacing pulses for delivering cardioversion anddefibrillation shocks and for monitoring the patient's heart rhythm.Pacemaker/cardioverter/defibrillator 10 is shown with the lead connectorassemblies 4, 13 and 14 inserted into the connector block 12, whichserves as a receptacle and electrical connector for receiving theconnectors 4, 13 and 14 and interconnecting the leads to the circuitrywithin housing 11. An optional sensor 30 is illustrated schematically bybroken outline, and may include one or more of an activity sensor,respiration sensor (potentially from impedance), accelerometer-basedposture detector, heart rate detector, ischemia detector and otheravailable physiological sensor known in the art for measuring hearthemodynamics and may be a piezoelectric transducer as known in the art.Sensor 30 may be used, for example, to regulate the underlying pacingrate of the device in rate responsive pacing modes.

Optionally, insulation of the outward facing portion of the housing 11of the pacemaker/cardioverter/defibrillator 10 may be provided or theoutward facing portion may instead be left uninsulated, or some otherdivision between insulated and uninsulated portions may be employed. Theuninsulated portion of the housing 11 optionally serves as asubcutaneous defibrillation electrode, used to defibrillate either theatria or ventricles. Other lead configurations and electrode locationsmay of course be substituted for the lead set illustrated. For example,atrial defibrillation and sensing electrodes might be added to eitherthe coronary sinus lead or the right ventricular lead instead of beinglocated on a separate atrial lead, allowing for a two lead system.

FIG. 2 is a functional schematic diagram of an implantablepacemaker/cardioverter/defibrillator of the type illustrated in FIG. 1,in which the present invention may usefully be practiced. This diagramshould be taken as exemplary of one type of anti-tachyarrhythmia devicein which the invention may be embodied, and not as limiting, as it isbelieved that the invention may usefully be practiced in a wide varietyof device implementations, including devices providing therapies fortreating atrial arrhythmias instead of or in addition to ventriculararrhythmias, cardioverters and defibrillators which do not provideanti-tachycardia pacing therapies, anti-tachycardia pacers which do notprovide cardioversion or defibrillation, and devices which deliverdifferent forms of anti-arrhythmia therapies such nerve stimulation ordrug administration.

The device is provided with a lead system including electrodes, whichmay be as illustrated in FIG. 1. Alternate lead systems may of course besubstituted. If the electrode configuration of FIG. 1 is employed, thecorrespondence to the illustrated electrodes is as follows. Electrode311 corresponds to an electrode formed along the uninsulated portion ofthe housing of the implantable pacemaker/cardioverter/defibrillator.Electrode 320 corresponds to electrode 20 and is a defibrillationelectrode located in the right ventricle. Electrode 310 corresponds toelectrode 8 and is a defibrillation electrode located in the coronarysinus. Electrode 318 corresponds to electrode 28 and is a defibrillationelectrode located in the superior vena cava. Electrodes 324 and 326correspond to electrodes 24 and 26, and are used for sensing and pacingin the ventricle. Electrodes 317 and 321 correspond to electrodes 19 and21 and are used for pacing and sensing in the atrium.

Electrodes 310, 311, 318 and 320 are coupled to high voltage outputcircuit 234. Electrodes 324 and 326 are coupled to the R-wave amplifier200, which preferably takes the form of an automatic gain controlledamplifier providing an adjustable sensing threshold as a function of themeasured R-wave amplitude. A v-sense signal is generated on R-out line202 whenever the signal sensed between electrodes 324 and 326 exceedsthe present sensing threshold.

Electrodes 317 and 321 are coupled to the P-wave amplifier 204, whichpreferably also takes the form of an automatic gain controlled amplifierproviding an adjustable sensing threshold as a function of the measuredR-wave amplitude. A signal is generated on P-out line 206 whenever thesignal sensed between electrodes 317 and 321 exceeds the present sensingthreshold. The general operation of the R-wave and P-wave amplifiers 200and 204 may correspond to that disclosed in U.S. Pat. No. 5,117,824, byKeimel, et al., issued Jun. 2, 1992, for an Apparatus for MonitoringElectrical Physiologic Signals, incorporated herein by reference in itsentirety. However, any of the numerous prior art sense amplifiersemployed in implantable cardiac pacemakers, defibrillators and monitorsmay also usefully be employed in conjunction with the present invention.

Switch matrix 208 is used to select which of the available electrodesare coupled to wide band amplifier 210 for use in digital signalanalysis. Selection of electrodes is controlled by the microprocessor224 via data/address bus 218, which selections may be varied as desired.Signals from the electrodes selected for coupling to bandpass amplifier210 are provided to multiplexer 220, and thereafter converted tomulti-bit digital signals by A/D converter 222, for storage in randomaccess memory 226 under control of direct memory access circuit 228.Microprocessor 224 may employ digital signal analysis techniques tocharacterize the digitized signals stored in random access memory 226 torecognize and classify the patient's heart rhythm employing any of thenumerous signal processing methodologies known to the art.

Telemetry circuit 330 receives downlink telemetry from and sends uplinktelemetry to the patient activator by means of antenna 332. Data to beuplinked to the activator and control signals for the telemetry circuitare provided by microprocessor 224 via address/data bus 218. Receivedtelemetry is provided to microprocessor 224 via multiplexer 220. Theatrial and ventricular sense amp circuits 200, 204 produce atrial andventricular EGM signals which also may be digitized and uplinktelemetered to an associated programmer on receipt of a suitableinterrogation command. The device may also be capable of generatingso-called marker codes indicative of different cardiac events that itdetects. A pacemaker with marker-channel capability is described, forexample, in U.S. Pat. No. 4,374,382 to Markowitz, incorporated byreference herein in its entirety. The particular telemetry systememployed is not critical to practicing the invention, and any of thenumerous types of telemetry systems known for use in implantable devicesmay be used. In particular, the telemetry systems as disclosed in U.S.Pat. No. 5,292,343 issued to Blanchette et al., U.S. Pat. No. 5,314,450,issued to Thompson, U.S. Pat. No. 5,354,319, issued to Wyborny et al.U.S. Pat. No. 5,383,909, issued to Keimel, U.S. Pat. No. 5,168,871,issued to Grevious, U.S. Pat. No. 5,107,833 issued to Barsness or U.S.Pat. No. 5,324,315, issued to Grevious, all incorporated herein byreference in their entireties, are suitable for use in conjunction withthe present invention. However, the telemetry systems disclosed in thevarious other patents cited herein which are directed to programmableimplanted devices, or similar systems may also be substituted. Thetelemetry circuit 330 is of course also employed for communication toand from an external programmer, as is conventional in implantableanti-arrhythmia devices.

The device of FIG. 2 includes an optional activity sensor 344, mountedto the interior surface of the device housing or to the hybrid circuitwithin the device housing and corresponds to sensor 30 of FIG. 1. Thesensor 344 and sensor present in circuitry 342 may be employed in theconventional fashion described in U.S. Pat. No. 4,428,378 issued toAnderson et al, incorporated herein by reference in its entirety, toregulate the underlying pacing rate of the device in rate responsivepacing modes.

The remainder of the circuitry is dedicated to the provision of cardiacpacing, cardioversion and defibrillation therapies, and, for purposes ofthe present invention may correspond to circuitry known in the priorart. An exemplary apparatus is disclosed for accomplishing pacing,cardioversion and defibrillation functions as follows. The pacertiming/control circuitry 212 includes programmable digital counterswhich control the basic time intervals associated with DDD, VVI, DVI,VDD, AAI, DDI, DDDR, VVIR, DVIR, VDDR, AAIR, DDIR and other modes ofsingle and dual chamber pacing well known to the art. Circuitry 212 alsocontrols escape intervals associated with anti-tachyarrhythmia pacing inboth the atrium and the ventricle, employing, any anti-tachyarrhythmiapacing therapies known to the art.

Intervals defined by pacing circuitry 212 include atrial and ventricularpacing escape intervals, the refractory periods during which sensedP-waves and R-waves are ineffective to restart timing of the escapeintervals and the pulse widths of the pacing pulses. The durations ofthese intervals are determined by microprocessor 224, in response tostored data in memory 226 and are communicated to the pacing circuitry212 via address/data bus 218. Pacer circuitry 212 also determines theamplitude of the cardiac pacing pulses under control of microprocessor224.

During pacing, the escape interval counters within pacer timing/controlcircuitry 212 are reset upon sensing of R-waves and P-waves as indicatedby signals on lines 202 and 206, and in accordance with the selectedmode of pacing on time-out trigger generation of pacing pulses by paceroutput circuits 214 and 216, which are coupled to electrodes 317, 321,324 and 326. The escape interval counters are also reset on generationof pacing pulses, and thereby control the basic timing of cardiac pacingfunctions, including anti-tachyarrhythmia pacing.

The durations of the intervals defined by the escape interval timers aredetermined by microprocessor 224, via data/address bus 218. The value ofthe count present in the escape interval counters when reset by sensedR-waves and P-waves may be used to measure the durations of R-Rintervals, P-P intervals, PR intervals and R-P intervals, whichmeasurements are stored in memory 226 and are used in conjunction withthe present invention to determine oversensing and in conjunction withtachyarrhythmia detection functions.

Microprocessor 224 operates as an interrupt driven device, and isresponsive to interrupts from pacer timing/control circuitry 212corresponding to the occurrences of sensed P-waves and R-waves andcorresponding to the generation of cardiac pacing pulses. Theseinterrupts are provided via data/address bus 218. Any necessarymathematical calculations to be performed by microprocessor 224 and anyupdating of the values or intervals controlled by pacer timing/controlcircuitry 212 take place following such interrupts. Microprocessor 224includes associated ROM in which the stored program controlling itsoperation as described below resides. A portion of the memory 226 may beconfigured as a plurality of recirculating buffers, capable of holdingseries of measured intervals, which may be analyzed in response to theoccurrence of a pace or sense interrupt to determine whether thepatient's heart is presently exhibiting atrial or ventriculartachyarrhythmia.

The arrhythmia detection method of the present invention may include anyof the numerous available prior art tachyarrhythmia detectionalgorithms. One preferred embodiment may employ all or a subset of therule-based detection methods described in U.S. Pat. No. 5,545,186 issuedto Olson et al. or in U.S. Pat. No. 5,755,736 issued to Gillberg et al.,both incorporated herein by reference in their entireties. However, anyof the various arrhythmia detection methodologies known to the art mightalso usefully be employed in alternative embodiments of the invention.

In the event that an atrial or ventricular tachyarrhythmia is detected,and an anti-tachyarrhythmia pacing regimen is desired, timing intervalsfor controlling generation of anti-tachyarrhythmia pacing therapies areloaded from microprocessor 224 into the pacer timing and controlcircuitry 212, to control the operation of the escape interval counterstherein and to define refractory periods during which detection ofR-waves and P-waves is ineffective to restart the escape intervalcounters.

In the event that generation of a cardioversion or defibrillation pulseis required, microprocessor 224 employs the escape interval counter tocontrol timing of such cardioversion and defibrillation pulses, as wellas associated refractory periods. In response to the detection of atrialor ventricular fibrillation or tachyarrhythmia requiring a cardioversionpulse, microprocessor 224 activates cardioversion/defibrillation controlcircuitry 230, which initiates charging of the high voltage capacitors246, 248 via charging circuit 236, under control of high voltagecharging control line 240. The voltage on the high voltage capacitors ismonitored via VCAP line 244, which is passed through multiplexer 220 andin response to reaching a predetermined value set by microprocessor 224,results in generation of a logic signal on Cap Full (CF) line 254,terminating charging. Thereafter, timing of the delivery of thedefibrillation or cardioversion pulse is controlled by pacertiming/control circuitry 212. Following delivery of the fibrillation ortachycardia therapy the microprocessor then returns the device tocardiac pacing and awaits the next successive interrupt due to pacing orthe occurrence of a sensed atrial or ventricular depolarization. In theillustrated device, delivery of the cardioversion or defibrillationpulses is accomplished by output circuit 234, under control of controlcircuitry 230 via control bus 238. Output circuit 234 determines whethera monophasic or biphasic pulse is delivered, whether the housing 311serves as cathode or anode and which electrodes are involved in deliveryof the pulse.

FIG. 3 is a flow chart of a method for delivering therapy in a medicaldevice, according to the present invention. As illustrated in FIG. 3, inorder to reduce the delivery of inappropriate therapy due tolead-related problems, a method for delivering therapy in a medicaldevice according to an embodiment of the present invention includesfirst predicting the presence of a lead-related condition, Block 340,and once the presence of a lead-related condition is detected,determining whether oversensing is likely taking place, Block 342. Ifoversensing is likely occurring, the delivery of therapy, such as shocktherapy, for example, is withheld, Block 346. On the other hand, ifoversensing is not likely occurring, normal delivery of the therapytakes place, Block 344. Thus, the present invention provides atwo-tiered approach for reducing delivery of shock due to lead-relatedconditions by combining an early warning prediction algorithm e.g., twoweeks prior to VF detection (though even after inappropriate shocks havebegun, the algorithm may still be useful to reduce further inappropriateshock)) and identification of inappropriate VF detection due tooversensing caused by a lead related condition such as oversensing orbased upon some other lead related parameter that may be predictive orindicative of lead failure or poor performance. For example, abnormalimpedance measurements may be indicative of a lead condition, thoughoversensing has not yet occurred After satisfying the predictionalgorithm, Block 340, selected parameters (e.g., EGM storage) could bechanged that are to be used by the inappropriate detection algorithm(e.g., RV coil to can EGM), Block 342. Therapy is subsequently withheldonly if both the prediction of the presence of a lead related conditionis satisfied and the determination that oversensing is likely takingplace is satisfied. It should be appreciated that if only one tier issatisfied, additional steps may be taken to gather additionalinformation as explained in greater detail hereinafter.

FIG. 4 is a flow chart of a method for detecting a lead-relatedcondition in a method for delivering therapy according to an embodimentof the present invention. As illustrated in FIG. 4, once a method fordetecting a lead-related condition (Block 340 of FIG. 3) is initiated, acriteria counter is set equal to zero, Block 700, and a determination ismade as to whether a first oversensing criteria is satisfied, such aswhether a sensing integrity counter has been satisfied, Block 702. Ifthe first oversensing criteria is satisfied, as will be described belowin detail, the criteria counter is incremented, Block 704. Once eitherthe first oversensing criteria is determined to have been satisfied andthe criteria counter has been incremented, or the first oversensingcriteria is determined not to be satisfied, a determination is made asto whether a second oversensing criteria, such as a non-sustained eventcounter, is satisfied, Block 706. If the second oversensing criteria issatisfied, as will be described in detail below, the criteria counter isincremented, Block 708. Once either the second oversensing criteria isdetermined to have been satisfied and the criteria counter has beenincremented, or the second oversensing criteria is determined not to besatisfied, a determination is made as to whether an impedance criteriadetermined using the methods described above is satisfied, Block 710. Ifthe impedance criteria is satisfied, the criteria counter isincremented, Block 712. Once either the impedance criteria is determinedto have been satisfied and the criteria counter has been incremented, orthe impedance criteria is determined not to be satisfied, adetermination is made as to whether more than one of the criteria hasbeen met, Block 714.

This paragraph number jumped from 21 to 39. If it is determined thatmore than one of the criteria have been met, such as both of theoversensing criteria or at least one of the oversensing criteria and theimpedance criteria, the likelihood of a lead-related condition issatisfied, and a determination as to whether oversensing is likelyoccurring is initiated, Block 717, which is described below in referenceto FIGS. 13-18. In addition to initiating the determination of thelikelihood of oversensing occurring, Block 717, a patient alert is alsotriggered, Block 716, once the determination that more than one of thecriteria have been met, YES in Block 714. According to the presentinvention, the patient alert may be implemented in an implantablemedical device implanted within the patient or may be implemented on anetwork server, as will be described below.

FIG. 5 is a flowchart of a method for determining whether an oversensingcriteria has been satisfied during detection of a lead-related conditionaccording to an embodiment of the present invention. As illustrated inFIG. 5, according to an embodiment of the present invention, in order todetermine whether the sensing integrity counter has been satisfied inBlock 702 in the method for detecting a lead-related condition of FIG.4, a next RR-interval is determined, Block 720, and a determination ismade as to whether the RR-interval is less than a predeterminedthreshold, Block 722. Since oversensing due to a lead related problemoften occurs near the blanking period of the sense amplifier, thesensing integrity counter quantifies this oversensing by counting thenumber of RR-intervals that are determined to be less than apredetermined time period above the blanking period, such as 20 ms abovethe blanking period, for example. Since, in certain devices, theblanking period is set as 120 ms, the predetermined threshold of Block722 would therefore be equal to approximately 140 ms, for example.According to an embodiment of the present invention, in devices in whichthe blanking period is programmable and can therefore have a value otherthan 120 ms, the predetermined threshold in Block 722 is simply setequal to the programmed blanking period plus 20 ms, with a maximum valueof 170 ms, for example. While the predetermined time that the thresholdis set above the blanking period is described as being 20 ms, it isunderstood that the present invention is not intended to be limited 20ms, but rather, would include using any other desired time period.

If the RR-interval is not less than the predetermined threshold, andtherefore is not near the blanking period, i.e., within 20 ms of theblanking period, a next RR-interval is obtained, Block 720, and adetermination is made as to whether the next RR-interval is less thanthe predetermined threshold, Block 722. Each time that the currentRR-interval is determined to be less than the predetermined thresholdand therefore near the blanking period, a counter is incremented, Block724. If the current RR-interval is the initial RR-interval determined tobe near the blanking period for the current session, i.e., the counteris equal to one in Block 726, a date/time stamp since the last sessionis obtained from the timing and control circuitry 212, for example, todetermine a start time of the current session, Block 728.

Once the session start time is determined, a next RR-interval isobtained, Block 720, and the process is repeated, with a determinationbeing made as to whether the next RR-interval is less than thepredetermined threshold, Block 722. If the current RR-interval is notthe initial RR-interval determined to be near the blanking period forthe current session, i.e., the counter is not equal to one in Block 726,a determination is made as to whether the number of RR-intervals thatare near the blanking period, i.e., less than the threshold in Block722, is equal to a predetermined threshold number, Block 730. If lessthan the predetermined threshold number of RR-intervals are near theblanking period, NO in Block 730, a next RR-interval is obtained, Block720, and the process is repeated, with a determination being made as towhether the next RR-interval is less than the predetermined threshold,Block 722.

Once the number of RR-intervals that are near the blanking period isequal to the predetermined threshold number, YES in Block 730, a currenttime window duration is determined by taking the difference between thestart time of the current session obtained in Block 728 and the currentdate/time stamp obtained from the timing and control circuitry 212,Block 732. Once the current time window duration is determined, adetermination is made as whether the current time window duration isless than or equal to a threshold time window, Block 734. If the timeduration window is less than or equal to the threshold time window, theoversensing criteria is determined to be satisfied, Block 736, andtherefore the criteria counter, Block 704 of FIG. 4, is incremented.

According to an embodiment of the present invention, the predeterminedthreshold number utilized in Block 730 is set equal to thirty and thethreshold time window is set equal to three days for Block 734, so thatone way in which the oversensing criteria is satisfied and therefore theoversensing criteria counter is incremented is if thirty RR-intervalsare determined to be near the blanking period within the first threedays, for example. However, any desired values for the predeterminedthreshold number of Block 730 and the threshold time window of Block 734without departing from the present invention. According to the presentinvention, the predetermined threshold number utilized in Block 730 isgiven a value corresponding to an indication that a mechanical problemassociated with the lead is present, such as a loose set screw, and istherefore set equal to thirty, for example, although any desired valuemay be utilized. In addition, although three days is utilized in Block734, any desired number of days or other time period may be utilized.

If the time duration window is greater than the threshold time window,NO in Block 734, a determination is made as to whether the number ofRR-intervals determined to be near the blanking period during thecurrent session is greater than a second predetermined threshold number,Block 738, by determining whether the counter in Block 724 is greaterthan the second predetermined threshold number of Block 738. Accordingto an embodiment of the present invention, the second predeterminedthreshold number of Block 738 is set as 300, for example, although anythreshold value may be chosen. If the number of RR-intervals near theblanking period is greater than the second threshold, the oversensingcriteria is determined to be satisfied, Block 736, and the criteriacounter, Block 704 of FIG. 4, is incremented. If the number ofRR-intervals near the blanking period is less than or equal to thesecond threshold, No in Block 738, a determination is made as to whetherthe time duration window is greater than a second threshold time period,such as 30 days, for example, Block 740.

If the time duration window is not greater than the second thresholdtime period, an average sensing integrity counter per day is determined,Block 742, by dividing the count of the number of RR-intervalsdetermined to be near the blanking period, Block 724, by the currenttime window duration determined in Block 732. A determination is thenmade as to whether the average sensing integrity counter per day isgreater than or equal to a predetermined threshold rate, such as 10 perday, for example, Block 744, although the predetermined threshold ratein Block 744 could have any desired value associated with an indicationof a lead-related condition. If the average sensing integrity counterper day is greater than or equal to the predetermined threshold rate,the oversensing criteria is determined to be satisfied, Block 736, andtherefore the criteria counter, Block 704 of FIG. 4, is incremented. Ifaverage sensing integrity counter per day is not greater than or equalto the predetermined threshold rate, a next RR-interval is obtained,Block 720, and the process is repeated with a determination being madeas to whether the next RR-interval is less than the predeterminedthreshold, Block 722.

Finally, if the number of RR-intervals near the blanking period is lessthan or equal to the second threshold, No in Block 738, and the timeduration window is greater than the second threshold time period, YES inBlock 740, a next RR-interval is obtained, Block 720, and the process isrepeated with a determination being made as to whether the nextRR-interval is less than the predetermined threshold, Block 722.

FIG. 6 is a graphical representation of a method for determining whetheran oversensing criteria has been satisfied during detection of alead-related condition according to an embodiment of the presentinvention. In this way, as illustrated in FIG. 6, a threshold TH fordetermining when the sensing integrity criteria is satisfied andtherefore the criteria counter of Block 704 of FIG. 4, is incremented,changes at predetermined time periods. In particular, for the firstthree days of the current session the threshold TH is satisfied oncemore than thirty RR-intervals are determined to be near the blankingperiod. Once three days have expired and there have not been thirtyRR-intervals determined to be near the blanking period, the threshold issatisfied once there are determined to be an average of ten RR-intervalsper day that are determined to be near the blanking period. After thirtydays have expired in the current session, the threshold is satisfiedonce there have been a total of three hundred RR-intervals determined tobe near the blanking period.

FIG. 7 is a flowchart of a method for determining whether an oversensingcriteria has been satisfied during detection of a lead-related conditionaccording to an embodiment of the present invention. As illustrated inFIG. 7, according to an embodiment of the present invention, a secondoversensing criteria utilized in the method for detecting a lead relatedcondition relates to identification of non-sustained episodes (Block 706of FIG. 4). During the normal detection process, the device detectsventricular tachyarrhythmias (VF, VT and FVT) by comparing timeintervals between sensed events to a set of programmable detectionintervals. For example, when the interval between sensed ventricularevents is between 320 and 400 ms, a ventricular tachycardia (VT)interval counter is incremented. Once a certain number events associatedwith the VT interval are detected, such as 16 events, for example, a VTevent is detected and the device responds appropriately.

In addition, a non-sustained ventricular tachycardia event is identifiedand stored within a non-sustained (NST) episode log when less than therequired number of events associated with the VT interval are detected,i.e., less than 16 events, but more than a predetermined number ofevents associated with the VT interval are identified, such as five forexample. The NST epsiode log stores information relating to thenon-sustained events, including a date/time stamp and an average cyclelength of each non-sustained episode. Because it has been determinedthat consecutive oversensed events may trigger storage of aninappropriate non-sustained episode in the NST episode log, the presentinvention utilizes the NST episode log as the second oversensingcomponent in detecting a lead-related condition, as described above inreference to FIG. 4.

For example, as illustrated in FIG. 7, in order to determine whether thesecond oversensing criteria has been satisfied in Block 706 of themethod for detecting a lead-related condition of FIG. 4, the averagecycle lengths for the non-sustained VT events stored in the NST episodelog are obtained, Block 760, and a determination is made as to whetherthere are a predetermined number of non-sustained VT events havingaverage cycle lengths that are less than a predetermined thresholdoccurring within a predetermined time frame, Block 762. For example,ventricular arrhythmia episodes typically have an average cycle lengthgreater than 200 ms and non-sustained episodes with average cyclelengths less than 200 ms are likely due to oversensing. Therefore,according to an embodiment of the present invention, a determination ismade in Block 762 as to whether two non-sustained VT events having acycle length less than 200 ms occur within a predetermined timeinterval, such as for example, a one week interval.

If the predetermined number of non-sustained VT events having averagecycle lengths that are less than the predetermined threshold occurwithin the predetermined time frame, the second oversensing criteria isdetermined to be satisfied, Block 764, and therefore the criteriacounter, Block 706 of FIG. 4, is incremented. If there are not thepredetermined number of non-sustained VT events having average cyclelengths that are less than the predetermined threshold occurring withinthe predetermined time frame, the second oversensing criteria isdetermined not satisfied, Block 766, and the process of detecting alead-related condition determines whether the other components aresatisfied.

FIG. 8 is a flow chart of monitoring impedance trends for the detectionand diagnosis of a lead-related condition according to an embodiment ofthe present invention. During the determination of impedance criteria(Block 710 of FIG. 4), for example, lead impedances are measured andstored, Block 355. Lead impedance measurements may be made on a periodicbasis, such as at least daily, for example. Multiple periodic impedancemeasurements may be made depending on the number of leads present andthe number of electrodes and conductors carried by each lead. For theconfiguration shown in FIG. 1, a preferred set of lead impedancemeasurements includes a low voltage pacing impedance measured across tipelectrode 26 and ring electrode 24 and high voltage impedances measuredacross: 1) ring electrode 24 and can 11, 2) ring electrode 24 and coilelectrode 20, 3) tip electrode 26 and coil electrode 20, and 4) tipelectrode 26 and can 11.

Based on measured and stored lead impedances, relatively short-termimpedance trend parameters are determined at Block 360, and relativelylong-term impedance trend parameters are determined at Block 365. Theseshort- and long-term impedance trend parameters are examined at decisionBlock 370 and 375 to determine if the trends are indicative of alead-related condition. This examination may include comparing aperiodic impedance measurement to impedance trend parameters todetermine if diagnostic criteria for detecting a lead-related conditionare met. If any of the examined trends are indicative of a leadcondition, the condition is diagnosed at Block 385 based on the trendanalysis. The diagnosed condition and supporting data may be stored inmemory 226 at Block 387 so that a clinician may upload this informationto an external device for review. A corrective action may optionally berecommended which may be to check for a loose connection between a leadand the associated IMD or replace a lead or add an additional lead whilecontinuing to use the functioning part of the old lead. At optionalBlock 390, patient notification signal may be generated so that thepatient is aware of a potential problem and seeks medical attention.

As a safety check in case of a sudden lead failure, a most recent leadimpedance measurement may be compared to an acceptable range at decisionBlock 380. An acceptable range may be a predefined range of impedancesknown to be normal for a particular lead type. If a single measurementis out of the acceptable range, a lead-related condition is diagnosed atBlock 385. If no trend or single impedance measurement indicate alead-related condition as determined at decision Block 370, 375 and 380,the method 350 may operate in a looping fashion by returning to Block355 to continue measuring and storing impedance data and updating theshort-term and long-term impedance trends at Blocks 360 and 365.

Periodic impedance measurements are performed by impedance measurementcircuit 204 under the control of microprocessor 224 and are stored inmemory 226 of ICD 10. In one embodiment, impedance measurement data maybe uplinked to an external device for analysis. Such data storage andtransmission is provided in commercially available devices, for examplein the GEM® Implantable Cardioverter Defibrillator available fromMedtronic, Inc., Minneapolis, Minn. Determination and analysis ofimpedance trend parameters for detecting a lead-related condition maythen be performed by an external device, which may be a programmer orpersonal computer. Uplinked impedance data may be alternatively betransferred via Internet to a central computer for analysis at a remotelocation. Reference is made to U.S. Pat. Appln. No. 20010031997 entitled“Instrumentation and software for remote monitoring and programming ofimplantable medical devices (IMDs)” to Lee, and U.S. Pat. Appln. No.20010037366 entitled “System and method for providing remote expertcommunications and video capabilities for use during a medicalprocedure” to Webb et al., both patent applications being incorporatedherein by reference in their entirety. Alternatively, impedance trendparameters may be determined by programs executed by microprocessor 224and stored in memory 226. Subsequent analysis of impedance trends may beperformed by microprocessor 224 or by an external device after uplinkinga history of impedance measurements and impedance trend parameters fromICD 10. The operations shown in FIG. 8 may be performed in real-time byICD 10 such that a lead-related condition may be detected early on andpatient notification signal may be generated to alert the patient toseek medical attention. The detected lead-related condition andsupporting data may then be uplinked to an external device for review bya physician, who may then take prompt action to confirm and correct theproblem.

The operations summarized in FIG. 8 are shown in greater detail in theflow charts of FIGS. 9A through 12. FIG. 9A is a flow chart of a methodfor determining short-term and long-term impedance trend parameters thatmay be included in an embodiment of the impedance trend monitoring ofFIG. 8. According to an embodiment of the present invention, impedancemeasurements are made at least daily and each daily impedancemeasurement is stored in memory 226 at Block 405. A given number ofdaily (or otherwise periodic) impedance measurements may be stored for apre-determined term, for example the most recent 14 daily impedancemeasurements may be stored as short-term impedance measurements fordetermining a relatively short-term impedance trend.

A relatively longer term is defined for determining long-term impedancetrends. According to an embodiment of the present invention, a long-termtrend stores weekly measurements over many weeks, such as 1 year forexample, or longer. The maximum impedance measurement measured over therelatively longer term and the minimum impedance measurement measuredover the relatively longer term are preferably determined as thelong-term maximum and long-term minimum impedances. In the method 400 ofFIG. 9A, a weekly maximum impedance is determined and stored at Block410, and a weekly minimum impedance is determined and stored at Block415.

From the stored daily (short-term) measurements and weekly (long-term)impedance parameters, short-term and long-term trends may be determined.For example, a short-term median impedance is determined at Block 420from a predetermined number of recent, consecutive periodicmeasurements. In one embodiment, the median of 14 daily impedancemeasurements is determined, for example.

FIG. 9B is a graphical representation of hypothetical daily impedancedata generated according to an embodiment of the present invention. Asillustrated in FIGS. 9A and 9B, fourteen daily impedance measurementsare plotted vs. time, from day 0 through 13 days prior, Block 405. Themedian daily impedance 440, indicated approximately by dashed line, isthen determined in Block 420 from the 14 daily impedance measurements tomonitor the trend of the measured short-term impedances. In addition, asdescribed above, a weekly minimum impedance measurement and a weeklymaximum impedance measurement is determined using the daily impedancedata generated for each week and stored in memory 226. The highestimpedance measurement 438 and the lowest impedance measurement 434 madeduring week 0 are stored as the weekly maximum and minimum impedancemeasurements, Blocks 410 and 415, respectively, for week 0. Likewise,the highest measurement 434 and lowest measurement 432 made during week−1 are stored as the weekly maximum and minimum impedances, Blocks 410and 415, respectively, for week −1.

At Block 425 of FIG. 9A, a maximum baseline impedance is determined fromstored long-term maximum impedance measurements, Block 410. Similarly, aminimum baseline impedance is determined, Block 430 from storedlong-term minimum impedance measurements, Block 415. In one embodiment,trends of long-term maximum impedances and long-term minimum impedancesare examined exclusively from each other. However, other algorithmscould be designed that combine both maximum and minimum impedances. Themaximum and minimum impedance measurements determined over a period oftime may deviate significantly from a median measurement if a short oropen has occurred along an impedance measurement pathway. For example,if a conductor fracture has occurred, a high impedance may be measured.The high impedance measurement may be intermittent, however, due tomotion of the lead body. Periodic impedance measurements for the samepathway, therefore, may continue to fall in a normal range, or close toa median, with occasional high maximum impedance. Intermittent highlong-term maximum impedances may therefore occur with a relativelystable minimum long-term impedance.

In another example, if a conductor insulation is breached, a lowimpedance measurement may occur. Again, a low impedance measurement maybe intermittent due to lead movement resulting in some periodicimpedance measurements to be relatively normal. Intermittent lowlong-term minimum impedances may occur with stable long-term maximumimpedance. Thus, the trends in the weekly maximum and minimum impedancesmay be different and mutually exclusive, depending on the type oflead-related condition that may be present. In accordance with thepresent invention, therefore, a maximum weekly baseline and a minimumweekly baseline are determined to allow mutually exclusive analysis oftrends in these parameters.

A long-term maximum baseline impedance, Block 425, and a long-termminimum baseline impedance, Block 430, may be determined based on thelong-term maximum and minimum impedance measurements over a given numberof terms. FIG. 9C is a graphical representation of a method fordetermining long-term maximum and minimum baselines according to anembodiment of the present invention. As illustrated in FIG. 9C, 8 weeksof maximum and minimum weekly impedance values, for example, are plottedvs. time, from week 0 through 7 weeks prior, although any number ofweeks may be utilized. A long-term maximum baseline impedance 441 isdetermined as the second lowest weekly maximum impedance determined fromthe 8 weekly maximum impedance measurements. A long-term minimumbaseline 442 is determined as the second highest weekly minimumimpedance determined from 8 weekly minimum impedance measurements. Byusing the second lowest and second highest maximum or minimum impedancemeasurement for setting a maximum or minimum baseline, respectively,outliers may be ignored. Long-term maximum and minimum baselineimpedances may alternatively be determined based on a median value ofthe maximum or minimum measurements, respectively, a percentage of amedian value, or other function of the long-term maximum and minimumimpedances. A new baseline is created each week using the most recenteight weeks, forming a sliding baseline window.

Other impedance trend parameters may be alternatively or additionally bedetermined such as impedance variability, slopes of short-term orlong-term impedance measurements versus time, etc. Once parametersrepresenting the short-term and long-term impedance trends have beenobtained, subsequent periodic impedance measurements may be compared tothese trend parameters to determine if a lead-related condition ispresent. Thus, method 400 of FIG. 9A may continue to method 450 of FIG.10.

FIG. 10 is a flow chart of a method of monitoring impedance to detect anopen or short circuit according to an embodiment of the presentinvention. As illustrated in FIG. 10, the decision Blocks 455, 460 and465 are included in an analysis 452 for determining if an open circuitis indicated. At Block 455, a long-term periodic impedance measurementis compared to a long-term maximum baseline measurement. According to anembodiment of the present invention, each subsequent daily impedancemeasurement is compared to the median daily impedance Block 440 and eachsubsequent weekly maximum impedance measurement is compared to theweekly maximum baseline impedance, Block 441, determined according tothe method 400 described above, so that subsequent daily measurementsare compared to the daily median and subsequent weekly measurements arecompared to the weekly baselines. If the current weekly maximumimpedance measurement is significantly greater than the weekly maximumbaseline, Block 441, for example 2 times greater, a counter that hasbeen previously initialized to 0 (not shown) is increased by one countat Block 457. Once the counter reaches a predetermined maximum asdetermined at decision Block 459, an open circuit is detected at Block470. In one embodiment, an open circuit is detected if three weeklymaximum impedance measurements exceed twice the weekly maximum baseline.

If the periodic impedance measurement does not significantly exceed thelong-term maximum baseline, or if the counter of Block 457 has notreached the predetermined maximum, the current short-term impedancemeasurement is compared to the short-term median impedance at decisionBlock 460. For example, according to the embodiment illustrated in FIG.9B, a daily impedance measurement is compared to the median of the 14most recent daily measurements. If the current short-term measurement issignificantly greater than the short-term median, for example more thantwice the short-term median, a counter that has been previouslyinitialized to 0 (not shown) is increased by one at Block 462. Once thecounter reaches a maximum, as determined at decision Block 464, an opencircuit is detected at Block 470. In one embodiment, an open circuit isdetected if three periodic short-term impedance measurements exceedtwice the short-term median impedance.

If the periodic measurement does not significantly exceed the short-termmedian impedance, or if the counter of Block 464 has not reached themaximum, the periodic short-term measurement is compared to a maximumacceptable impedance, at Block 475, which may be a fixed, predeterminedvalue or a programmable value selected based on the type of lead used.In one embodiment, an open circuit is detected at Block 470 if the dailypacing impedance measurement exceeds 2000 ohms.

Thus an open circuit may be detected based on a single impedancemeasurement being outside of a predetermined range associated witheither a median daily impedance or a single daily impedance threshold,or, in accordance with the present invention, based on a short-term orlong-term impedance trend. Diagnostic criteria set for detecting alead-related condition based on comparisons between a periodic impedancemeasurement and short-term and long-term impedance parameters may betailored to a particular lead type. For example, the difference betweena periodic lead measurement and an impedance parameter trend and thenumber of periodic measurements deviating significantly from animpedance parameter trend may be uniquely defined depending on the typeof lead being monitored. Upon detection of an open circuit, the method350 of FIG. 8 will store the lead-related condition along with thesupporting data that led to the detection (Block 385) and may provide arecommended corrective action or generate a patient notification signal.

If an open circuit is not detected during the open circuit analysis,Block 452, the method 450 proceeds to Block 475 to begin an analysis 454for detecting a short circuit, which includes the decision Blocks 475,480 and 485. At Block 475, a long-term periodic impedance measurement iscompared to a long-term minimum baseline measurement. According to anembodiment of the present invention, a subsequent weekly minimumimpedance measurement is compared to the weekly minimum baselineimpedance determined according to the method 400 described above. If theweekly minimum impedance measurement is significantly less than theweekly minimum baseline impedance, Block 442, for example less than halfthe weekly minimum baseline impedance, a counter, that has beenpreviously initialized to 0 (not shown) is increased by one count atBlock 477. Once the counter reaches a predetermined maximum asdetermined at decision Block 479, a short circuit is detected at Block490. In one embodiment, a short circuit is detected if three dailyimpedance measurements are less than half the weekly minimum baseline.

If the periodic impedance measurement is not significantly less than thelong-term minimum baseline, or the counter of Block 477 has not reachedthe maximum, the current short-term impedance measurement is compared tothe short-term median impedance at decision Block 480. For example,according to the embodiment of FIG. 9B, a daily impedance measurement iscompared to the median of the 14 most recent daily measurements. If thecurrent short-term measurement is significantly less than the short-termmedian, for example less than half the short-term median, a counter thathas been previously initialized to 0 (not shown) is increased by one atBlock 482. Once the counter reaches a maximum, as determined at decisionBlock 484, a short circuit is detected at Block 490. In one embodiment,a short circuit is detected if three daily impedance measurements areless than half the short-term median impedance.

If the periodic measurement is not significantly less than theshort-term median impedance, or if the counter of Block 48 has notreached the maximum, the periodic measurement is compared to a minimumacceptable impedance, which may be a fixed, predetermined value or aprogrammable value, at Block 485. In one embodiment, if the daily pacingimpedance measurement is less than 200 ohms, an open circuit is detectedat Block 470. Upon detection of a short circuit, method 350 of FIG. 8will store the diagnosis and supporting data (Block 385) in memory 226an optionally provide a recommended corrective action or generate apatient notification signal.

If an open or short circuit is not detected by method 450 of FIG. 10,the method 450 returns to Block 400 of FIG. 9A to collect the nextperiodic impedance measurement, update the trend parameters accordingly,and continue to test for a lead-related condition in a looping fashion.Tests for a lead-related condition may further include a more rigorousanalysis of long-term trends to detect a gradually occurring condition.

FIG. 11A is a flow chart of a method of monitoring impedance to detectinsulation degradation according to an embodiment of the presentinvention. According to the present invention, a gradual degradation ofthe outer insulation of a lead body may be detected by monitoringimpedance trends over a relatively long-term. Method 500 begins at Block505 by determining the running median of a given number of consecutivelong-term minimum impedance measurements. In one embodiment, the medianis determined from 5 weekly minimum impedance measurements. The runninglong-term median is then determined for a given number of terms. Forexample, a five-week median may be determined for 12 weeks. Next,parametric linear regression is performed on the 12 five-week medianvalues at Block 510. The slope of the linear regression, which may be aleast squares fit, is then compared to a minimum acceptable slope atdecision Block 515. If a negative slope is found that represents adecrease in the impedance over the 12-week period of greater than apredetermined percentage, X, for example 30%, then a lead degradationproblem is suspected. If the comparison made at decision Block 515 isnot affirmed, the method 500 returns to Block 505 to continuedetermining a running median of the weekly minimum impedance andperforming the linear regression analysis at Block 510.

If the comparison at Block 515 is affirmed, then a decline in impedancedue to a lead replacement must be excluded before concluding that leaddegradation condition exists. A single “step-wise” decrease in leadimpedance can occur when a lead has been replaced. Therefore, to verifythat the overall decrease is not due to a step-wise decrease associatedwith a lead replacement, the difference between each of the consecutivefive-week median values used in the parametric analysis is determined atBlock 517. If two consecutive medians differ by greater than apredetermined amount, for example greater than 35%, as determined atdecision Block 520, then a lead replacement has occurred as concluded atBlock 530. If consecutive median differences do not indicate a step-wisechange, then the gradual decrease in the running median impedance isconcluded to be due to insulation degradation at Block 525. Thisdiagnosis and the supporting data are stored in memory 226 at Block 385of method 350 (FIG. 8) for later uplinking to an external device forphysician review, and a recommended action and/or a patient notificationsignal may be generated as described previously.

FIG. 11B is flow chart of a method for detecting lead insulationdegradation using non-parametric methods according to an embodiment ofthe present invention. In method 550, the long-term minimum impedancemeasurement is determined for a desired number of terms at Block 555. Ina preferred embodiment, a weekly minimum impedance is determined for 12weeks. At Block 560, the successive differences between the long-termminimum impedances are determined. At decision Blocks 565 and 570, anon-parametric analysis is performed to determine if the successivedifferences indicate a gradually decreasing trend of the long-termminimum impedance. In one embodiment, a given number, N, successivedifferences must be negative with no more than a given number, M,successive differences being positive wherein N should be greater thanM. In one embodiment, if successive differences between 12 weeklyminimum impedance measurements have been determined, at least fivesuccessive differences must be negative, as determined at decision Block565, and no more than two successive differences may be positive, asdetermined at Block 570, in order to diagnose a lead insulationdegradation condition at Block 575. If the diagnostic requirements ofthe non-parametric analysis are not met at decision Block 565 and 570,the next long-term minimum impedance and associated successivedifference is determined at Block 580. Method 550 then returns to Block565 to continue monitoring the successive differences to determine ifthe diagnostic requirements are met.

The method 450 of FIG. 10 for detecting an open or short circuit andmethods 500 or 550 of FIGS. 11A and 11B for detecting insulationdegradation represent general methods that may generally be applied tomany lead types. Supplementary analyses of impedance trends may beperformed for detecting lead-related conditions that are characteristicof a particular lead type.

One lead related condition that can occur with certain types of leads isdegradation of a middle insulation layer due to metal ion oxidation.This type of degradation is observed in leads having coaxially arrangedconductors separated by polyurethane insulation. This phenomenon is notobserved in other types of leads, such as leads having conductorsarranged in a multi-lumen, silicone rubber lead body. Therefore,supplementary analysis of impedance trend data may include an analysisfor detecting and diagnosing metal ion oxidation induced degradation. Ina preferred embodiment, the type of lead in which lead impedancemeasurements are being made is preferably known so that appropriatesupplementary analyses may be made. The lead type may be enteredmanually as a lead model number upon implantation by the physician. Ifthe lead type is not known, supplementary analyses preferably includetests that will exclude types of leads that would not be subject to theparticular type of lead-related condition being investigated.

FIG. 12 is a flow chart of a method for monitoring trends in leadimpedance parameters to detect middle insulation degradation due tometal ion oxidation according to an embodiment of the present invention.Because this type of lead-related condition is specific to certain leaddesigns, method 600 begins at decision Block 601 to determine if thelead model in which lead impedance measurements are being made is known.If the lead model is not known, the method 600 may continue with theanalyses but preferably includes steps for excluding leads not subjectto metal ion oxidation (MIO) as will be described below.

If the lead model is known, method 600 determines if the model issubject to MIO at decision Block 602. The known lead model number may becompared to a list of lead model numbers known to be subject to MIO. Ifthe lead model is not subject to MIO, method 600 is terminated at Block603. If the lead is subject to MIO, method 600 continues to Block 605 tobegin analyzing impedance trends.

In order to specifically diagnose middle insulation degradation, thetrend of multiple lead impedance parameters is monitored. At Block 605long-term impedance parameters are determined for multiple impedancemeasurement pathways. Middle insulation degradation due to MIO istypically observed in true bipolar defibrillation leads havingpolyurethane insulation between a coil electrode and a ring electrode.When this insulation layer begins to degrade, the impedance pathwayalong any pathway that includes the ring electrode and/or the coilelectrode is affected. At Block 605, multiple long-term high-voltage(HV) impedance parameters are determined for a predetermined number ofterms, N. The parameters preferably include a long-term minimum acrossring and coil electrodes, a long-term minimum across the coil and canelectrodes, and a long-term maximum across the ring and coil electrodes.In a preferred embodiment, the long term is a term of one week, andweekly parameters are collected for seven weeks.

At decision Blocks 610, 620 and 630, three criteria for diagnosingmiddle insulation degradation due to MIO are tested. The firstcriterion, tested at decision Block 610, is that a given number M, ofthe N long-term minimum ring-to-coil impedances must be less than anacceptable level, which would indicate a short between the ring and coilelectrodes due to degradation of the intervening insulation. In apreferred embodiment, any four out of seven consecutive weekly minimumring-to coil impedances must be lower than 14 ohms. If this criterion isnot met, method 600 continues to Block 615 to determine the nextlong-term impedance parameters which will be stored in a rolling memorybuffer designated for storing the most recent N parameters. Afterstoring the new weekly parameters, the tests for MIO are repeated.

If the first criterion at decision Block 610 is satisfied, the secondcriterion is tested at decision Block 620. The second criterion is thateach long-term minimum coil-can impedance is greater than apredetermined percentage of the median minimum coil-can impedancedetermined from the N terms. In a preferred embodiment, each weeklyminimum coil-can impedance must be greater than 50% of the median ofseven consecutive weekly minimum coil-can impedances. If any of theweekly minimum coil-can impedances is less than half of the medianminimum coil-can impedance, then a short of the outer coil insulationmay be present. An outer insulation problem will be detected anddiagnosed by the methods described previously for detecting a short orgeneral insulation degradation. When the second criterion is not met,the method 610 proceeds to Block 615 to determine the next long-termimpedance parameters and will continue to monitor the impedanceparameters according to the MIO diagnostic criteria.

If the second criterion is met, thereby ruling out that the decrease inthe ring-coil minimum impedances found at decision Block 610 is not dueto an outer insulation breach of the coil electrode, middle insulationdegradation to MIO is likely to be present. The final criterion, testedat decision Block 630, is included in the case that the lead modelnumber is not known. If the lead model number is not known, the lead inwhich impedances are being measured may be an integrated bipolar leadrather than a true bipolar lead. Middle insulation degradation due toMIO has not been observed in an integrated bipolar lead. Therefore, thethird criterion is provided to establish that the lead is not anintegrated bipolar lead.

The ring-coil impedances measured in an integrated bipolar lead will beconsiderably lower than the ring-coil impedances measured in a truebipolar lead. Therefore one way to discriminate between an integratedand true bipolar lead is to monitor the maximum long-term ring-coilimpedance. If this maximum remains in a lower range, typical of anintegrated bipolar lead, then the lead is known to be an integratedbipolar lead, generally not subject to MIO, rather than a true bipolarlead. Conversely, if the maximum long-term ring-coil impedance remainsin a higher range, associated with a true bipolar lead, then the lead isknown to be a true bipolar lead that is subject to MIO.

At decision Block 630, a median of a desired number of maximum long-termring-coil impedances is compared to a predetermined maximum limit thatis considered an upper boundary for the maximum ring-coil impedance ofan integrated bipolar lead. In a preferred embodiment, the median ofseven weekly maximum ring-coil impedances must be less than 5 ohms ifthe lead is an integrated bipolar lead. If this comparison is true, thelead is known to be an integrated bipolar lead as indicated at Block640. No middle insulation condition is diagnosed.

If the comparison at decision Block 630 is not true, then the finalcriterion for diagnosing middle insulation degradation due to MIO in atrue bipolar lead is satisfied as indicated at Block 635. This diagnosisand supporting data may be stored in memory 225 and a recommendedcorrective action, which would generally be lead replacement or additionof a ventricular pace/sense lead, may be indicated. A patientnotification signal may be generated.

Thus, a lead-specific condition, such as middle insulation degradationdue to MIO, may be diagnosed by monitoring multiple lead impedancemeasurement trends. This supplementary monitoring of impedance trendsmay be performed in addition to monitoring one or more individual leadimpedance measurement trends for diagnosing general lead-relatedconditions associated with sudden or gradually occurring short or opencircuits.

FIG. 13 is a portion of a stored electrogram showing near-field andfar-field pulses where there is an indication of a false positivenear-field pulse. As illustrated in FIG. 13, the near-field signal 40 isrecorded between the tip and ring electrodes of the bipolar sensinglead, such as electrodes 24 and 26, for example. This signal is input toa sense amplifier that senses voltages that exceed a threshold. Thefar-field signal 42 is recorded between secondary electrodes such as thelead coil and the can or a sensing lead in another part of the heart(left auricle or right ventricle). A marker channel 43 below far-fieldelectrogram 42 displays each sensed event from the near-field signal,such as Fibrillation Sense (FS), Fibrillation Detected (FD), TachycardiaSensed (TS) Ventricular Sense (VS) Capacitors charged (CE), or CapacitorDischarged (CD) for example. The numbers below the letters on markerchannel 43 indicate the time between sensed events. For example on theleft side of FIG. 3 there are two VS events, and the number below andbetween them is “670”, indicating that there were 670 millisecondsbetween the two VS events. Note that at the left of the electrogram wave40 is a relatively normal R-wave representation 44. The period ofrelative normal R-wave representation 44 is followed by a series oferratic signals 46 that indicate an oversensing problem (i.e., afractured lead conductor or insulation break on the lead).

An examination of far-field signal 42, however, shows a relativelyregular far-field R-wave. During the period of relative normal R-waverepresentation 44, the far-field signal 42 follows the near-field signal40 quite closely. When the near-field signal 40 becomes erratic in anerratic portion 46, the far-field signal 42 continues to show regularR-wave far-field pulses indicating that the erratic portion 46 may bedue to oversensing. As the near-field signal 40 recovers at a period ofrelative normal R-wave representation 48, the far-field signal 42continues to follow the near-field signal 40, suggesting that theirregular portion 46 of the near-field signal 40 was due to oversensing,and probably an intermittent failure, since the R-wave pulses ofnear-field signal 40 recovered at a period of relative normal R-waverepresentation 48.

With a pattern of this nature, it would be premature to deliver atherapy to the patient, particularly a painful defibrillation shock, inresponse to erratic portion 46 sensed in far-field signal 40. Typicallyseveral methods are used to avoid delivering a shock under theseconditions. First, if there is a detection of an irregularity as seen inthe erratic portion 46 of near-field signal 40, one can wait to seewhether the problem goes away by increasing the number of intervals fordetection (as is the case in the waves of FIG. 13), which would suggestthat the problem may be an oversensing problem and not an arrhythmia.Also, the sensing lead electrode configuration could be changed, andpacemakers may be programmed to automatically change the sensing leadconfiguration (e.g. bipolar to unipolar). Finally, the patient could begiven an alert (a vibration or audio alert, for example) to advise thepatient to see his doctor to have the ICD and its leads checked, but analert would not prevent the shock at the moment of oversensing.

FIG. 14 is a portion of an electrogram showing near-field and far-fieldR-wave sensing pulses where there is an actual cardiac episode requiringtherapy. The near-field signal 40 and the far-field signal 42 are shownas in FIG. 13. In this case the beginning (left side) of near-fieldsignal 40 shows relatively normal R-waves in portion 50, although thepulses are inverted from those of FIG. 13. Likewise far-field signal 42confirms the regularity during portion 50. At portion 52 of thenear-field signal 40, however, a highly irregular waveform exists.Unlike in FIG. 3, however, the far-field wave 42 does not maintain aregular R-wave periodicity during portion 50, but rather confirms theirregularity of near-field signal 40. This would strongly suggest anarrhythmia in the patient's ventricle and call for therapy in the formof a defibrillation shock. As above, however, certain intermediate stepsmay be taken before actually administering the shock such as waiting ashort period of time (perhaps ten or fifteen seconds) to see whether thesituation resolves itself. This period of time occurs because thecapacitors are charging. If in fact this waveform identifies anarrhythmia event, a therapy must be administered very quickly.

The decision to administer a therapy has been based primarily upon thenear-field R-wave. The present invention uses the far-field electrogramto discriminate QRS complexes between supraventricular (e.g. sinustachycardia, atrial fibrillation) and ventricular arrhythmias. In thisway, the present invention provides an algorithm that takes into accountother information to provide a better determination of an actualarrhythmia before subjecting a patient to a painful defibrillationshock.

FIG. 15 is a flow chart of a method for determining the presence ofoversensing in a method for of delivering a therapy in an implantablemedical device, according to an embodiment of the present invention.Once the lead-related condition has been detected, as described above,the subsequent determination of whether oversensing is taking place(Block 342 of FIG. 3) is initiated by beginning sensing between theidentified sensing electrodes for near-field and far-field sensing. Inparticular as illustrated in FIG. 14, each time a V-sense signal issensed between a near-field sensor, i.e., electrodes 24 and 26corresponding to a next beat, Block 800, a determination is made as towhether the sensed event is a VF event, with a counter corresponding tothe number of sensed events and number of VF events being incremented inorder to generate a number of intervals for detection of ventricularfibrillation (VF NID), Block 802. In either case, i.e., the event is nota VF event or the event is determined to be a VF event, a determinationis made as to whether a predetermined number of VF events M have beendetected, Block 804, by determining whether the number of intervals fordetection of ventricular fibrillation is greater than the predeterminednumber M. If the predetermined number of VF events M has not beendetected, NO in Block 804, the process waits for the next beat to occur,Block 800. Once the predetermined number of VF events M have beendetected, YES in Block 804, a baseline measure associated with afar-field signal associated with the beat that is sensed betweensecondary electrodes is determined, Blocks 806-812, as described below.The secondary electrodes for sensing the far-field signal can includethe lead coil 20 and the uninsulated portion of the housing 11, forexample, or a sensing lead 6, 15 in another part of the heart alone orin combination with the uninsulated portion of the housing 11. Inaddition, the far-field sensing electrodes could also include one of thenear-field electrodes.

According to one embodiment of the present invention, for example, thepredetermined number M is set as three events so that once three VFevents are detected, a baseline measure is determined, Blocks 806-812,for each subsequently sensed beat.

FIGS. 16A and 16B are graphical representations of a determination of abaseline measure of a far-field signal according to an embodiment of thepresent invention. In particular, as illustrated in FIGS. 15, 16A and16B, in order to determine the baseline measure for the sensed beatsoccurring after the predetermined number of VF events M have beendetected, amplitude values associated with a predetermined number ofsamples located within a predetermined window 840 of a far-field signal842 that is centered around the sensed beat 844 are determined, Block806. According to an embodiment of the present invention, window 840 isset as a 188 ms window, for example, so that amplitude values aredetermined for 12 samples using a 64 Hz sampling rate.

Both a variance, such as a standard deviation SD, for example, of theamplitudes of the 12 samples, Block 808, and a central tendency, such asa mean, a median, or a sum, for example, of the absolute values of theamplitudes of the twelve samples is determined, and so that the baselinemeasure is calculated using the product of the variance and the centraltendency of the absolute values, Block 812.

In addition to calculating a baseline measure associated with thecurrent sensed beat, a determination is made as to whether the VF NID isgreater than a predetermined threshold, Block 814. For example,according to an embodiment of the present invention, a determination ismade in Block 814 as to whether 18 out of the last 24 beats weredetermined to be VF events. If the VF NID threshold is not reach, NO inBlock 814, the process waits for the next beat to be sensed, Block 800.Once the VF NID threshold has been met, YES in Block 814, adetermination is made as to whether the baseline measure determined forany of a predetermined number of previously sensed beats, such as thelast twelve beats, for example, is less than a predetermined threshold,Block 816. If none of the baseline measures associated with thepredetermined number of previously sensed beats is less than thepredetermined threshold, NO in Block 816, no oversensing is determinedto be occurring and VF detection is confirmed, Block 818. If any one ofthe baseline measures associated with the predetermined number of sensedbeats is less than the threshold, YES in Block 816, oversensing isdetermined to likely be occurring, Block 820, such as would result froma lead integrity failure, for example, and a patient alert, such as avibration or audio alert, a wireless signal transmitted to a remotemonitor, satellite, internet, for example, is activated to alert thepatient, Block 822, and to advise the patient to see his doctor to havethe ICD and its leads checked. Because the alert is not intended toprevent the shock at the moment of oversensing, the process is repeatedfor the next beat, Block 800, until a predetermined time periodassociated with the charging of the capacitor(s) for delivering theshock, such as 10 seconds, for example, has expired, YES in Block 824.Once the timer has expired, the VF detection process for delivering acorresponding shock therapy continues according to the normal VFdetection process, Block 826, and therapy is delivered as determinednecessary.

According to an embodiment of the invention, once it is determined thatthe patient is experiencing a VF event, i.e., the VF NID is greater thanthe threshold (18 of last 24 beats are determined to be VF events) andoversensing is determined, the device begins charging of one or morecapacitors for delivering the therapy to the patient. According toanother embodiment of the invention, once it is determined thatoversensing is likely to be occurring, charging of the capacitors may bewithheld until the timer of Block 824 has expired, for example.

FIGS. 16A and 16B are graphical representations of a determination of abaseline measure of a far-field signal according to an embodiment of thepresent invention. As illustrated in FIGS. 16A and 16B, in order toreduce the effects of oversensing, the present invention evaluates thecorresponding far-field signal to determine whether a VF episode is alsoindicated in the far-field signal, so that if the VF NID threshold ismet in Block 816 due to oversensing rather than the occurrence of a VFepisode, such as when there is a loss in lead integrity resulting fromlead fractures, corrupted connector interfaces, EMI issues, R-waveoversensing, myopotentials, etc., the patient is alerted of the possibleoversensing issue. In particular, when VF is detected in the signalsensed by the near-field sensor but not in the signal detected by thefar-field sensor 842, the standard deviation of the amplitudes of thesamples in the associated window 840 and the absolute values of theamplitudes will be negligible since the far-field signal will likelyapproach the isoelectric baseline value of the far-field EGM signal, asillustrated in FIG. 16A. However, when VF is detected both in the signalsensed by the near-field sensor and in the signal detected by thefar-field sensor 842, the standard deviation of the amplitudes of thesamples in the associated window 840 and the absolute values of theamplitudes will be greater. Therefore the product of the standarddeviation and the sum of the absolute values, Block 812, will be smallwhen there is oversensing compared to when an actual arrhythmia event isoccurring. In particular, according to an embodiment of the presentinvention, the threshold of Block 816 is set equal to one, so that ifthe product of the standard deviation and the sum of the absolute valuesis less than one for any of the predetermined number of previouslysensed events, it is likely that the isoelectric baseline of thefar-field signal is occurring and therefore oversensing is likelyoccurring.

FIG. 17 is a flow chart of a method for determining the presence ofoversensing in a method for delivering a therapy in a medical device,according to an embodiment of the present invention. As illustrated inFIG. 17, according to an embodiment of the present invention each time aV-sense signal is sensed between a near-field sensor, i.e., electrodes24 and 26 corresponding to a next beat, Block 900, a determination ismade as to whether the sensed event is a predetermined event, such as aVF event, for example, with a counter corresponding to the number ofsensed events and number of VF events being incremented in order togenerate a number of intervals for detection of the predetermined event,i.e., ventricular fibrillation (VF NID), Block 902. In either case,i.e., the event is not a VF event or the event is determined to be a VFevent, a determination is made as to whether a predetermined number ofVF events M have been detected, Block 904, by determining whether thenumber of intervals for detection of ventricular fibrillation is greaterthan the predetermined number of VF events M. According to oneembodiment of the present invention, for example, the predeterminednumber of VF events M is set as three events.

If the predetermined number of VF events M has not been detected, NO inBlock 904, the process waits for the next beat to occur, Block 900. Asillustrated in FIGS. 17, 16A and 16B, once the predetermined number ofVF events M have been detected, YES in Block 904, amplitude valuesassociated with a predetermined number of samples located within apredetermined window 600 located over a portion of the far-field signal842 and centered around the current sensed beat 844 are determined,Block 906. Window 840 is set as a 188 ms window, for example, so thatamplitude values are determined in window 840 for 12 samples using a 64Hz sampling rate to generate 12 amplitude values 848 associated witheach window 840.

Once the 12 amplitude values 848 have been determined, a sampleamplitude is generated for the current sensed event, Block 908, bydetermining a maximum amplitude and a minimum amplitude of the 12amplitude values 848 for the far-field window 840 associated with thesensed event, and determining the difference between the maximumamplitude and the minimum amplitude. The process is repeated for thenext sensed beats, generating a sample amplitude for each of thesubsequently sensed events. Once the sample amplitude is generated forthe sensed beat, a determination is made as to whether an episoderequiring therapy, such as ventricular fibrillation for example, isdetected by determining whether the VF NID is greater than apredetermined threshold, Block 910. For example, according to anembodiment of the present invention, an episode requiring therapy isdetermined to be present in Block 910 when 18 out of the last 24 beatsare determined to be VF events. If the VF NID threshold is not reach, NOin Block 910, the process waits for the next beat to be sensed, Block900 and is repeated for the next sensed beat.

Once an episode requiring therapy is detected, i.e., the VF NIDthreshold has been met, YES in Block 910, a maximum sample amplitude anda minimum sample amplitude associated with a predetermined number of thesensed events 846, such as 8 sensed events, for example, is determined,Block 912. Although the predetermined number of sensed events 846illustrated in FIG. 16A is shown as including eight sensed events, it isunderstood that the maximum sample amplitude and a minimum sampleamplitude of any desired number of sensed events could be utilized andthe invention is not intended to be limited to the use of eight sensedevents. A determination is then made as to whether the maximum sampleamplitude is greater than a predetermined maximum sample amplitudethreshold, such as 2 mv for example, and the minimum sample amplitude isless than a predetermined minimum sample amplitude threshold, such as 1mv for example, Block 914.

If the maximum sample amplitude is not greater than the predeterminedmaximum sample amplitude threshold or the minimum sample amplitude isnot less than the predetermined minimum sample amplitude threshold, NOin Block 914, the process waits for the next beat to be sensed, Block900, and is then repeated for the next sensed event. If the maximumsample amplitude is greater than the predetermined maximum sampleamplitude threshold and the minimum sample amplitude is less than thepredetermined minimum sample amplitude threshold, YES in Block 914, adetermination is made as to whether the minimum sample amplitude is lessthan a predetermined percentage of the maximum sample amplitude, Block916. For example, according to an embodiment of the present invention,the determination in Block 916 involves determining whether the minimumsample amplitude is less than one sixth of the maximum sample amplitude,although any desired percentage may be utilized. It is understood thatalthough 2 mv and 1 mv are utilized as the maximum and minimum sampleamplitude thresholds, respectively, any desired value may be utilizedfor the predetermined maximum and minimum sample amplitude thresholds.

If the minimum sample amplitude is greater than or equal to apredetermined percentage of the maximum sample amplitude, NO in Block916, no oversensing is determined to be occurring and VF detection isconfirmed, Block 918. If the minimum sample amplitude is less than apredetermined percentage of the maximum sample amplitude, YES in Block916, oversensing is determined to likely be occurring, Block 920, suchas would result from a lead integrity failure, for example, and apatient alert, such as a vibration or audio alert, a wireless signaltransmitted to a remote monitor, satellite, internet, for example, isactivated to alert the patient, Block 922, and to advise the patient tosee his doctor to have the ICD and its leads checked. Because the alertis not intended to prevent the shock at the moment of oversensing, theprocess is repeated for the next beat, Block 900, until a predeterminedtime period associated with the charging of the capacitor(s) fordelivering the shock, such as 10 seconds, for example, has expired, YESin Block 924. Once the timer has expired, the VF detection process fordelivering a corresponding shock therapy continues according to thenormal VF detection process, Block 926, and therapy is delivered asdetermined necessary.

As described above, according to an embodiment of the invention, once itis determined that the patient is experiencing a VF event, i.e., the VFNID is greater than the threshold (18 of last 24 beats are determined tobe VF events) and oversensing is determined, the device begins chargingof one or more capacitors for delivering the therapy to the patient.According to another embodiment of the invention, once it is determinedthat oversensing is likely to be occurring, charging of the capacitorsmay be withheld until the timer of Block 924 has expired, for example.

FIG. 18 is a graphical representation of maximum and minimum amplitudesof sensed events in a method of determining the presence of oversensingin a method for delivery of therapy in a medical device according to anembodiment of the present invention. In the graphical representation ofFIG. 18, both actual VT/VF events 860 from a VT/VF episode and non-VT/VFevents 804 that are most likely the result of oversensing caused byinstances of lead failure, for example, sensed by the far-fieldelectrode configuration are plotted on a graph of maximum amplitudes (X)versus minimum amplitudes (Y). As can be seen in FIG. 18, the non-VT/VFevents 862 tend to occur within an area 864 defined by boundariescorresponding to the maximum sample amplitude being greater that 2 mv866, the minimum sample amplitude being less that 1 mv 868, and theminimum sample amplitude being less than one sixth of the maximum sampleamplitude 870, as described above.

It is understood that while the determination of the sample amplitude inBlock 908 is described as being generated by determining the differencebetween a maximum and a minimum amplitude of the 12 amplitude values848, the present invention may utilize other methods of determining thesample amplitudes and therefore is not intended to be limited to thedetermination of a maximum and a minimum amplitude as described. Forexample, according to an embodiment of the present invention, the sampleamplitude may be determined in Block 908 using the secondlargest/smallest amplitude, or the third largest/smallest amplitude,etc., or may include looking at a range between values and so forth.

It is understood that other methods for detecting the presence of alead-related condition or the presence of oversensing in Blocks 340 and342 of FIG. 3, respectively, may be utilized, such as those set forth incommonly assigned U.S. patent application Ser. No. 10/436,626, filed May13, 2003, entitled “Identification of Oversensing Using Sinus R-WaveTemplate”, incorporated herein by reference in it's entirety.

Some of the techniques described above may be embodied as acomputer-readable medium that includes instructions for a programmableprocessor such as microprocessor 224 or pacer timing/control circuitry212 shown in FIG. 2. The programmable processor may include one or moreindividual processors, which may act independently or in concert. A“computer-readable medium” includes but is not limited to any type ofcomputer memory such as floppy disks, conventional hard disks, CD-ROMS,Flash ROMS, nonvolatile ROMS, RAM and a magnetic or optical storagemedium. The medium may include instructions for causing a processor toperform any of the features described above for actively determining acoupling interval according to the present invention.

One embodiment of the present invention provides a system and method forautomatically identifying and trouble-shooting cardiac and/ornon-cardiac oversensing by an implantable cardiac stimulation device.The methods included in the present invention may be used in conjunctionwith, or incorporated in, an implantable cardiac stimulation device suchas a pacemaker or an implantable cardioverter defibrillator (ICD), orother monitoring devices, capable of storing sensed intracardiacelectrogram (EGM) data including the intervals between sensed events(i.e. RR interval).

An exemplary ICD 1000 is shown in FIG. 19, which may be the same orsimilar to that of FIG. 1. ICD 1000 identifies oversensing andautomatically provides a corrective action, e.g., adjusts one or moredetection parameters to avoid future inappropriate detections.Particularly, ICD 1000 operates in accordance with originally programmedsensing and detection parameters for a plurality of cardiac cycles, andupon detecting oversensing, automatically provides the corrective actionto avoid possible future inappropriate detections. In this manner, thecorrective actions provided by ICD 1000 to avoid future inappropriatedetections are dynamically performed.

The ICD 1000 is shown coupled to a heart of a patient by way of threeleads 1006, 1015, and 1016. A connector block 1012 receives the proximalend of a right ventricular lead 1016, a right atrial lead 1015 and acoronary sinus lead 1006, used for positioning electrodes for sensingand stimulation in three or four heart chambers. In FIG. 19, rightventricular lead 1016 is positioned such that its distal end is in theright ventricle for sensing right ventricular cardiac signals anddelivering pacing or shocking pulses in the right ventricle. For thesepurposes, right ventricular lead 1016 is equipped with a ring electrode1024, an extendable helix electrode 1026 mounted retractably within anelectrode head 1028, and a coil electrode 1020, each of which areconnected to an insulated conductor within the body of lead 1016. Theproximal end of the insulated conductors are coupled to correspondingconnectors carried by bifurcated connector 1014 at the proximal end oflead 16 for providing electrical connection to the ICD 1010.

The right atrial lead 1015 is positioned such that its distal end is inthe vicinity of the right atrium and the superior vena cava. Lead 1015is equipped with a ring electrode 1021 and an extendable helix electrode1017, mounted retractably within electrode head 1019, for sensing andpacing in the right atrium. Lead 1015 is further equipped with a coilelectrode 1023 for delivering high-energy shock therapy. The ringelectrode 1021, the helix electrode 1017 and the coil electrode 1023 areeach connected to an insulated conductor with the body of the rightatrial lead 1015. Each insulated conductor is coupled at its proximalend to a connector carried by bifurcated connector 1013.

The coronary sinus lead 1006 is advanced within the vasculature of theleft side of the heart via the coronary sinus and great cardiac vein.The coronary sinus lead 1006 is shown in the embodiment of FIG. 19 ashaving a defibrillation coil electrode 1008 that may be used incombination with either the coil electrode 20 or the coil electrode 23for delivering electrical shocks for cardioversion and defibrillationtherapies. In other embodiments, coronary sinus lead 1006 may also beequipped with a distal tip electrode and ring electrode for pacing andsensing functions in the left chambers of the heart. The coil electrode1008 is coupled to an insulated conductor within the body of lead 1006,which provides connection to the proximal connector 1004.

The electrodes 1017 and 1021 or 1024 and 1026 may be used as truebipolar pairs, commonly referred to as a “tip-to-ring” configuration.Further, electrode 1017 and coil electrode 1020 or electrode 1024 andcoil electrode 1023 may be used as integrated bipolar pairs, commonlyreferred to as a “tip-to-coil” configuration. In accordance with theinvention, ICD 1010 may, for example, adjust the electrode configurationfrom a tip-to-ring configuration, e.g., true bipolar sensing, to atip-to-coil configuration, e.g., integrated bipolar sensing, upondetection of oversensing in order to reduce the likelihood of futureoversensing. In other words, the electrode polarities can be reselectedin response to detection of oversensing in an effort to reducesusceptibility of oversensing. In some cases, electrodes 1017, 1021,1024, and 1026 may be used individually in a unipolar configuration withthe device housing 1011 serving as the indifferent electrode, commonlyreferred to as the “can” or “case” electrode.

The device housing 1011 may also serve as a subcutaneous defibrillationelectrode in combination with one or more of the defibrillation coilelectrodes 1008, 1020 or 1023 for defibrillation of the atria orventricles. It is recognized that alternate lead systems may besubstituted for the three lead system illustrated in FIG. 19. While aparticular multi-chamber ICD and lead system is illustrated in FIG. 19,methodologies included in the present invention may adapted for use withany single chamber, dual chamber, or multi-chamber ICD or pacemakersystem, or other cardiac monitoring device.

In FIG. 20 a flow diagram is shown providing an overview of theoperations included in a preferred embodiment of the present inventionfor reducing the likelihood of inappropriate detection of noise in animplantable medical device. As illustrated in FIG. 20, a determinationis made to whether a lead failure is likely present, Block 1395, usinglead failure analysis such as the lead failure detection methoddescribed above. Lead failure may result in oversensing. According to anembodiment of the present invention, once a lead failure is determinedlikely to be present, yes in Block 1400, the programmed number ofintervals to detection (NID) is increased, Block 1402, and an alert isgenerated to notify the patient of the possible presence of the leadfailure, Block 1404. In addition to the patient alert, the device mayalso transmit a wireless alert to remotely notify a clinician. Once theNID has been increased, a timer is initiated so that once the increasedNID has been utilized for a programmed time period, such as four daysfor example up to the life of the device, Yes in Block 1406, the devicereturns from the increased NID setting to a reduced setting, Block 1408,such as the previously programmed NID setting, for example.

It has been determined that increasing the NID, when clinicallyacceptable, dramatically reduces the risk of an inappropriate shock dueto a lead failure. For example, it has been determined that increasingthe number in intervals for VF detection from 12 out of 16 intervals fordetection, as is commonly utilized, to 18 out of 24 intervals fordetection, the number of inappropriate shocks delivered due to the leadfailure is typically reduced by 46%. When the NID is increased to 24 outof 32 intervals for detection, the number of inappropriate shocksdelivered due to the lead failure is typically reduced by 81%, and whenthe NID is increased to 30 out of 40 intervals for detection, the numberof inappropriate shocks delivered due to the lead failure is typicallyreduced by 85%.

The detailed descriptions of the preferred embodiments provided hereinyield a sensitive and specific method for diagnosing oversensing ofcardiac or non-cardiac signals and automatically adjusting detectionparameters to reduce the likelihood of the device inappropriatelydetecting noise. Numerous variations of the described embodiments arepossible for practicing the invention. Therefore, the embodimentsdescribed herein should be considered exemplary, rather than limiting,with regard to the following claims. These and other embodiments arewithin the scope of the following claims.

1. A method comprising: operating an implanted medical device inaccordance with sensing parameters for a plurality of cardiac cycles;identifying oversensing by the implanted medical device (IMD); andautomatically increasing a programmed number of intervals to detection(NID) in response to identifying the oversensing.
 2. The method of claim1, wherein identifying oversensing includes predicating a lead failure.3. The method of claim 1, wherein identifying oversensing includesdetecting a lead failure.
 4. The method of claim 1, further comprisingtransmitting an alert to a patient indicative of the identifiedoversensing.
 5. The method of claim 1, further comprising transmittingan alert to a patient indicating that oversensing due to a lead failurehas been identified.
 6. The method of claim 1, further comprisingtransmitting via a wireless RF communication link an alert to aphysician indicating that oversensing has been identified by the IMD. 7.The method of claim 1, further comprising transmitting an alert to aphysician indicating that the NID of the IMD has been increased.
 8. Themethod of claim 1, further comprising initiating a timer for apredetermined duration in response to the automatic increase of the NID,wherein upon expiration of the timer the NID value is adjusted.
 9. Themethod of claim 8, wherein the predetermined duration is 96 hours. 10.The method of claim 8, wherein upon expiration of the timer the N ID isadjusted to the programmed value.
 11. The method of claim 8, whereinupon expiration of the timer the NID is adjusted to a value between theincreased value and the programmed value.
 12. The method of claim 1,wherein the NID is increased from the programmed value to requiringdetection in 18 out of 24 events.
 13. The method of claim 1, wherein theNID is increased from the programmed value to requiring detection in 24out of 32 events.
 14. The method of claim 1, wherein the NID isincreased from the programmed value to requiring detection in 30 out of40 events.
 15. An implantable medical device (IMD) comprising: means foroperating an implantable medical device in accordance with sensingparameters for a plurality of cardiac cycles; means for identifyingoversensing by the implantable medical device; and means forautomatically increasing a programmed number of intervals to detection.16. The IMD of claim 15, wherein identifying oversensing includespredicating a lead failure.
 17. The IMD of claim 15, wherein identifyingoversensing includes detecting a lead failure.
 18. The IMD of claim 15,further comprising means for transmitting an alert to a patientindicative of the identified oversensing.
 19. The IMD of claim 15,further comprising means for transmitting an alert to a patientindicating that oversensing due to a lead failure has been identified.20. The IMD of claim 15, further comprising means for transmitting via awireless RF communication link an alert to a physician indicating thatoversensing has been identified by the IMD.
 21. The IMD of claim 15,further comprising means for initiating a timer for a predeterminedduration in response to the automatic increase of the NID, wherein uponexpiration of the timer the NID value is adjusted.
 22. The IMD of claim21, wherein upon expiration of the timer the NID is adjusted to a valuebetween the increased value and the programmed value.
 23. The IMD ofclaim 15, wherein the NID is increased from the programmed value torequiring detection in 18 out of 24 events.
 24. The IMD of claim 15,wherein the NID is increased from the programmed value to requiringdetection in 24 out of 32 events.
 25. The IMD of claim 15, wherein theNID is increased from the programmed value to requiring detection in 30out of 40 events.